Transmission electron microscopy (TEM) is a known method of analyzing air filter samples for collected fibers and other particulates. Typically, the specimens are prepared for TEM by the Jaffe wick method in which membrane air filters (either of the polycarbonate or mixed cellulose ester type) containing collected particulate matter are first coated with carbon (on the particulate matter side) in a carbon evaporator. The carbon coated filter is then cut into small sections which are placed over the openings of a copper grid. The grid is placed horizontally on a sponge which is situated in a glass dish. An organic liquid which will dissolve the filter (chloroform for polycarbonate filters and acetone for mixed cellulose ester filters) is added to the dish until the liquid is level with the top surface of the sponge and thus in contact with the copper grid and the bottom surface of the filter. The liquid gradually dissolves the filter leaving behind the carbon film containing the particulate matter. The grid and carbon film can then be used as the specimen for the TEM analysis to determine the amount and nature of the particulate matter.
The Jaffe wick method is time consuming. In addition, the polycarbonate membrane filters do not always dissolve completely away, thus making the TEM image blurred and obscuring fibers and particles. Attempts to improve the Jaffe wick method, for example, by increasing the duration of the membrane dissolving step, simply have not resulted in an acceptable method.
TEM specimens have been prepared by floating of a carbon film from a substrate with particles entrapped. This technique has been used primarily to reproduce the structure of steel surfaces. The use of a carbon film on metal samples is described by Bradley and Williams, J. Gen. Microbiology, Vol. 17 (1957). However, it was not considered a technique for preparing specimens from soft and textured surfaces such as membrane filters.
Kornienko et al., Industrial Lab. (USA), Vol. 45, No. 3 (published September 1979) teaches a method for rapid preparation of replicas utilizing a celluloid film wetted in acetone and placed against the specimen. After drying, the film is detached from the specimen and a scratch on the surface of the specimen is clearly visible on the film. Subsequently, a platinum carbon replica is applied to the contact surface in a vacuum. The primary replica is then dissolved in acetone and the replica is recovered on a screen.
Japanese Publication 59-34132 teaches a method for preparation of a TEM replica sample in which Au, Pt or Pd is vapor-deposited on a polished surface by vacuum vapor-deposition.
U.S. Pat. No. 2,875,341 is of interest to show a method for preparation of replica specimens utilizing a coating of a plastic material such as polystyrene and benzol.